The present invention generally relates to a semiconductor device and a method of manufacturing a semiconductor device by which the performance of a heterojunction field effect transistor can be enhanced easily.
In recent years, the progress of technology in the information and communication field has been remarkable. As for the frequency bands dealt with in communication apparatuses, in communication circuits for processing high-frequency signals from the microwave band to the millimeter wave band, ON/OFF type switches for passing signals therethrough as well as route changeover type switches for controlling transmission lines by a combination of a plurality of the ON/OFF type switches and the like are used in many cases. The ON/OFF type switches constituting the basis are generally composed of mechanical switches for performing mechanical opening and closing actions, PN diode switches using the PN junction of a semiconductor, FET switches using the switching function of an FET (Field Effect Transistor), and the like (see, for example, Japanese Patent Laid-open No. 2001-110817).
The transmission/reception changeover RF switch for antennas in the past is applied to radio communication apparatuses designed to communicate at the same communication frequency fc. In the two-way communication of cellular phones, vehicle phones and the like, the FM (Frequency Modulation) modulation system or the PM (Pulse Modulation) modulation system is generally adopted. When an FM modulated signal is subjected to high-speed switching by such a switching device as FET, a signal distortion is generated due to the phase shift of frequency components in the FM modulated signal. Particularly, in the case of FET, the non-linearity of the internal capacitance in the OFF state is greater as compared with the internal resistance in the ON state, so that a large signal distortion is generated during the OFF period; moreover, the distortion is larger as the input power is greater. Therefore, when a signal distortion is generated in a transmitted signal, the noise power in the sideband waves is increased, giving bad influence on the reception in the adjacent channels; on the other hand, when a signal distortion is generated in a received signal, it causes intermodulation or cross modulation.
As for enhancement of the performance of the transmission/reception changeover RF switches, efficient signal changeover can be achieved by lowering the resistance at the time when the FET is turned ON and/or by lowering the capacity at the time when the FET is OFF. In view of this, technical developments have been under way for achieving a lower ON resistance and/or a lower OFF capacity under the condition of a single gate width. However, although a lower ON resistance can be attained by shortening of the drain-source distance or the like means, the shortening of the drain-source distance or the like means leads adversely to an increase in the OFF capacity due to the mutual approaching of the electrodes. Thus, the lowering of the ON resistance and the lowering of the OFF capacity are generally in a trade-off relationship. An increase in the OFF capacity leads to a lower impedance at high frequencies, thereby causing an increase in leakage current and an increase of intermodulation distortion. In relation to this point, the approach of reducing the parasitic capacity by size reduction has its limits under the situations of the existing technology.